- 206 THE HISTORY OF CREATION. 
sac, and a spleen. Further, all Double-nostriled animals 
possess a bladder-shaped expansion of the gullet, which, in 
Fish, has developed into the swimming bladder, but in all 
other Double-nostriled animals into lungs. Finally, in all 
Double-nostriled animals there exist in the youngest stage 
of growth the beginnings of two pairs of extremities, or 
limbs, a pair of fore legs, or breast fins, and a pair of hinder 
legs, or ventral fins. One of these pairs of legs sometimes 
degenerates (as in the case of eels, whales, etc.), or both 
pairs of legs (as in Ceecilize and serpents) either degenerate 
or entirely disappear; but even in these cases there exists 
some trace of their original beginning in an early embryonic 
period, or the useless remains of them may be found in the 
form of rudimentary organs. (Compare above, vol. i. p. 13.) 
From all these important indications we may conclude 
with full assurance that all double-nostriled animals are 
derived from a single common primary form, which 
developed either directly or indirectly during the primordial 
period out of the Monorrhina. This primary form must 
have possessed the organs above mentioned, and also the 
beginning of a swimming bladder and of two pairs of legs 
or fins. It is evident, that of all still living double-nostriled 
animals, the lowest forms of sharks are most closely allied 
to this long since extinct, unknown, and hypothetical 
primary form, which we may call the Primary Double- 
nostriled animals (Proselachii). We may therefore look 
upon the group of primeval fish, or Selachii, to which the 
Proselachit. probably belonged, as a primary group, not 
only of the Fish class, but of the whole main-class of double- 
nostriled animals. 
The class of Fish (Pisces) with which we accordingly 
